At best, the determination and attribution of global- mean sea level change lies at the very edge of knowledge and technology. The most urgent job would appear to be the accurate determination of the smallest temperature and salinity changes that can be determined with statistical significance, given the realities of both the observation base and modeling approximations. Both systematic and random errors are of concern, the former particularly, because of the changes in technology and sampling methods over the many decades, the latter from the very great spatial and temporal variability implied by Figs. 2, 6, and 8. It remains possible that the database is insufficient to compute mean sea level trends with the accuracy necessary to discuss the impact of global warming—as disappointing as this conclusion may be. The priority has to be to make such calculations possible in the future.

As the person who wrote the original post lo these many years ago, I must say that this has to rate as one of the most honest assessments of the state of the science that I have seen.

Since that time, the data have become available on the internet, making my digitization unnecessary. It is worth noting that the recent data shows a levelling off of the rise. Given the history, I suspect this will not last. But it clearly indicates that the many projections of an increasing rate of sea level rise have no observational support.

The map shown above illustrates another important point in the discussion. The tides have a very long and complex cycle. It is well known that no matter how accurate your measurements are, you need about a half century of data to give you an accuracy of ± 1mm in sea level rise. This is because of the non-repeating cycle of the tides, which only return to near (not exactly) where they started after about 50 years. In addition, there is a “sloshing” of the water in the various oceanic basins. As a result, our current satellite record of 17 years is far from adequate for any analysis of long term trends.

Finally, I am quite curious about the accuracy of the maps produced using the satellite data. While they are accurate in some areas, in others they seem to be unsupported by the local tidal data. Anthony has a discussion of one example at WUWT. It is worth noting in this context that to measure sea level to the nearest 1 mm, the measurement must be done to an accuracy of one part per billion. This is difficult to do even in a laboratory, much less from a satellite with a host of potential errors (satellite elevation, size of footprint, atmospheric delay, instrument stability over time, etc.).

Sea floor movement correction to global sea level is referred to as Glacial Isostatic Adjustment (see here for example). Land covered by glaciers kilometers thick at the end of the last ice age was depressed causing a corresponding increase in the sea floor height. Now that those glaciers are gone, the surface is rebounding, causing a corresponding reduction in sea floor height. 30 km3 of water seems like a lot but it’s vanishingly small compared to the volume of the earth ocean as a whole.

I wonder what effect the “ring of fire” is having on the detected sea level. From Wikipedia

Q//As well as the sideways movement between the plates, the sea floor is estimated to have risen by several metres, displacing an estimated 30 km3 (7 cu mi) of water and triggering devastating tsunami waves. The waves did not originate from a point source, as was inaccurately depicted in some illustrations of their paths of travel, but rather radiated outwards along the entire 1,600 km (994 mi) length of the rupture (acting as a line source). This greatly increased the geographical area over which the waves were observed, reaching as far as Mexico, Chile, and the Arctic. The raising of the sea floor significantly reduced the capacity of the Indian Ocean, producing a permanent rise in the global sea level by an estimated 0.1 mm (0.01 cm or 0.0001 m).[14]//unQ

Accounted for or not?? Too many of the hot spots are in areas of repeating plate movemnets.
regards

Hi Geoff, thanks for the reference, and don’t overlook this one on the 1950-2003 period, ” Two-dimensional reconstruction of past sea level (1950–2003) from tide gauge data and an Ocean General Circulation Model”. Interestingly they do not give an average sea level increase over the period (which overlaps by 10 years the period of the paper you referenced) buy eyeballing the graphs it doesn’t look like much more than 1.5 mm/year. In the new paper, they claim 3.11 mm/year for the whole period 1993-2008, but only 1 mm/year for 2005-8.

Looks like there is still work to be done and Wunsch’s comment that “the determination and attribution of global-mean sea level change lies at the very edge of knowledge and technology” still stands.